Recently, considering environmental issues, a hybrid vehicle that runs based on efficient combination of an engine and a motor has been put into practical use. Such a hybrid vehicle includes a power storage unit that can be charged or discharged and generates drive force by supplying electric power to a motor at the time of start or acceleration while it recovers kinetic energy of the vehicle as electric power during running down a slope or during braking. Therefore, a nickel metal hydride battery, a lithium-ion battery or the like adapted to large input/output electric power and charge/discharge capacity has been adopted as the power storage unit included in a hybrid vehicle.
A configuration called “plug-in” allowing charge/discharge of a power storage unit by using external power supply such as commercial power supply has been proposed for such a hybrid vehicle. The plug-in configuration aims to enhance overall fuel consumption efficiency by driving a relatively short distance, for example for commuting or shopping, with electric power stored in advance in the power storage unit from the external power supply while the engine is maintained in a non-operating state.
In a running mode using only electric power from the power storage unit, that is, in what is called an EV (Electric Vehicle) running mode, steady output of electric power is necessary. Accordingly, a charge/discharge capacity greater than that of a power storage unit included in a normal hybrid vehicle is required in the power storage unit in the plug-in configuration, whereas input/output electric power thereof may be relatively small.
Thus, in a hybrid vehicle adapted to the plug-in configuration, power storage units different in performance are necessary. Therefore, a configuration including a plurality of power storage units different in a charge/discharge characteristic is desirable. Regarding a configuration incorporating a plurality of power storage units, for example, U.S. Pat. No. 6,608,396 discloses a power control system providing desired high DC voltage levels required by a high voltage vehicle traction system. The power control system includes a plurality of power stages for providing DC power to at least one inverter, each stage including a battery and boost/buck DC-DC converter, the power stages wired in parallel, and a controller controlling the plurality of power stages so as to maintain a voltage output to at least one inverter by causing uniform charge/discharge of the batteries of the plurality of power stages.
In general, the power storage unit stores a relatively large amount of electric energy. Accordingly, from the viewpoint of safety, the power storage unit is always monitored for a fault condition based on a status value of the power storage unit. For example, a degree of deterioration is determined based on an internal resistance value of the power storage unit. If determination as fault is made, the power storage unit should electrically be disconnected from the system.
In the power control system disclosed in U.S. Pat. No. 6,608,396 described above, no attention is paid to a case where a fault condition occurs in a battery (power storage unit), and a configuration for electrically disconnecting the power storage unit where a fault condition occurs is not disclosed. Therefore, if only one of a plurality of power storage units is in the fault condition, the entire system should inevitably be stopped.